Temperature-dependent Raman study of the pyrochlore superconductor Cd 2Re 2O 7

Abstract

We present the results of Raman studies of the pyrochlore superconductor Cd 2Re 2O 7 as a function of temperature. The overall features of the observed Raman spectra are in accord with the prediction of the factor group analysis and clearly identify two successive structural transitions, one at 200 K and the other at 120 K. New modes at 156, 325, and 411 cm -1 appear below 200 K, and peaks at 193-200 cm -1 and at 345 cm -1 are detected below 120 K. Moreover, the peak near 676 cm -1 shows an abrupt softening by ∼20 cm -1 below 200 K, but a slow hardening behavior below 120 K. On the other hand, the slope of the full width at half maximum of the peak at 230 cm -1 is found to decrease abruptly below 200 K, which appears to be parallel with the changes in the electric properties. This is indicative of the close relation between the lattice dynamics and the electronic properties. Our Raman spectra suggest that the structural changes at 200 K and 120 K in the ReO(1) network, where O(1) is for oxygen atom at the apex of the ReO 6 octahedron, is responsible for the changes in the electronic properties of the Cd 2Re 2O 7 compound.